Consequence of unbalance supplying condition on a distribution static compensator

A static compensator (STATCOM) is composed of an AC/DC converter connected to a power system through a transformer to regulate voltage or compensate load reactive power. The applied unbalanced voltages from a distribution feeder would affect the behavior of the converter, both the harmonic performance and converter efficiency. Here we present different practical results, taken from an installed ±250 kVA STATCOM, showing the quick variations of the capacitor voltage at twice the synchronous frequency. In fact, this is an energy oscillation between the DC and AC sides, making waveforms distorted by introducing low order harmonics to the inverter output voltages and currents. This work, initially, analyzes the unbalance issue, while experimental results demonstrate the validity of the performed analysis. Then, a possible solution is suggested to eliminate the energy oscillation of STATCOM, confirming it by presenting satisfactory practical results.